Electrosurgery is the application of a high frequency electrical current to a surgical site on a patient for the purpose of tissue cutting and/or coagulation. The high frequency current that is generated by an ESU is applied to the patient's body from an active electrode which is held by the surgeon, and is collected from the patient's body at a dispersive electrode. A relatively small contact area of the active electrode to the tissue causes a high current density entering the patient at the surgical site. This high current density causes intense localized heating, arcing and other effects, to achieve the cutting and/or coagulation. The dispersive electrode collects the current from the patient's body and returns it to the ESU to complete the electrical circuit. The dispersive electrode is of a significant size so that the density of the current it collects is low enough to avoid any surgical or heating effect.
If the current in the patient's body ever develops a high current density during its passage through the body, localized tissue heating will occur and a burn will develop. This situation can occur if the current is allowed to exit the patient's body at a location other than the dispersive electrode. Such current is known as a leakage current. A burn from leakage currents can be quite severe as the patient is anesthetized and cannot react to the burn. The burn area is frequently covered so the doctor or surgical attendants will not see it until it is too late to take corrective action. Another potential for leakage current burns is to the surgeon from the active electrode or the conductors which supply the high frequency, high voltage electrosurgical energy. Leakage currents in this case may harm or burn the surgeon or one of the surgical attendants in contact with the active electrode or its supply conductor. It is for this reason that leakage or alternate path currents in electrosurgery are of considerable concern.
The first ESUs were of a ground-referenced design. Being ground referenced, the return for the ESU and the dispersive electrode were both connected to earth ground. The ground referenced arrangement was satisfactory provided that no other point on the patient was ground-referenced. For example, if a monitoring electrode was on the patient's body during the surgical procedure, and the monitoring electrode is referenced to ground, some portion of the electrosurgical current would flow to ground through the monitoring electrode, instead of the preferred path back through the dispersive electrode. Since monitoring electrodes may be small, the current density through them may be sufficient to develop a high enough density to cause a burn. An even worse condition occurs if the generator connection to the dispersive electrode is accidentally broken. With no direct current path back to the ESU, all of the electrosurgical current will travel through alternate grounded paths, such as through the monitoring electrodes and the surgical table, and severe burning is likely to result.
In an effort to reduce the risks associated with the ground-referenced ESUs, the power output circuit of the ESU was isolated from ground. Output isolated ESUs were a significant step in reducing the risks associated with alternate path burns, because the electrosurgical current exiting the patient was more likely to flow through the dispersive electrode than any ground referenced points on the patient, in returning to the ESU. If the generator connection to the dispersive electrode became disconnected, a significant amount of the electrosurgical current flow from the ESU would stop.
Although an improvement over the previous ground-referenced ESUs, the problem with isolated output ESUs was that the isolation from ground was not perfect. At the relatively high frequencies of electrosurgical current, e.g., 500 kiloHertz to 1 megaHertz, any stray capacitance to ground presents a ground referenced signal path. Furthermore, the amount of stray capacitance required to create a significant path for ground-referenced currents is not great. Although alternate path currents are less than those flowing if the ESU was ground-referenced, the potential still remains for significant patient and alternate path burns.
An improvement to help minimize alternate path currents in isolated ESUs involved the use of a differential transformer in the output circuit, as shown in U.S. Pat. No. 4,437,464. The electrosurgical current supplied to the active electrode flows through one winding on the transformer core, and the current from the dispersive electrode flows through the other winding. When the currents in the two windings are equal, as would be the case when no alternate path currents flow, the flux from both currents cancels, and transformer presents very little insertion loss or impedance to the flow of electrosurgical current. If a significant alternate path current does flow, the imbalance creates a flux in the core of the differential transformer causing a large insertion loss. The insertion loss increases the impedance and reduces the amount of current flowing to the active electrode. Thus, the current flow to the patient is reduced, also causing a decrease in the alternate path or leakage current. Although this approach reduces leakage current, it may not be sufficient to reduce the leakage current below a maximum acceptable safe level, for example one hundred fifty milliamps.
Another improvement which provides an alarm under conditions of excessive leakage current with isolated ESUs, or which terminates the delivery of all electrosurgical power, is disclosed in U.S. Pat. No. 3,683,923. A sensing winding on the differential transformer senses the imbalance in the current flowing in the active lead and the return lead. Upon sensing a sufficient imbalance in the current sensed, an alarm circuit is triggered and a warning is given to the operator. In addition, a relay may be simultaneously or alternatively activated to terminate the flow of current to the patient. The operator must then take corrective action such as reducing the power level or attempting to eliminate the problem causing current leakage, as well as reactivating the ESU.
It is against this background that the further significant improvements and advancements of the present invention have evolved in the field of controlling leakage current in isolated ESUs.